The present invention regards a semiconductor integrated inertial sensor with calibration microactuator.
As is known, the possibility of exploiting machinery and manufacturing processes typical of the microelectronics industry enables semiconductor integrated inertial sensors to be manufactured at a low cost, at the same time guaranteeing high reliability in terms of performance.
Although these inertial sensors are advantageous from various points of view, they present the drawback that their calibration is very complex, as well as costly, in that it is difficult to calibrate them at a wafer level.
In addition, the inertial sensors thus obtained have an offset and output/input sensitivity that depends upon the parameters of the process of fabrication, and consequently must be suitably calibrated.
In order to calibrate the sensor, one first known solution involves shaking of the inertial sensor, already inserted in its own package, on an electrodynamic or piezoelectric actuator validated according to required standards. The choice of a particular type of actuator is assessed on the basis of the range of the operating frequencies of the inertial sensor that is to be calibrated. The calibration curve that is obtained is then, generally, stored in a memory device formed in the die in which the inertial sensor itself is made. Even though this first known solution is advantageous from various points of view, it presents the drawback that it is extremely difficult to achieve at the wafer level.
A second known solution is described in U.S. Pat. No. 5,621,157, which envisages integration on one and the same wafer of the inertial sensor to be calibrated and of an electrostatic actuator, which simulates the unknown inertial quantity to be measured, and has the following characteristics:
it is linear in the voltage applied;
it is precise; i.e., its operation is practically independent of the parameters of the integration process.
xe2x80x9cPractically independentxe2x80x9d means that the electrostatic actuator has a configuration which is less sensitive than the inertial sensor is to the variations in the integration process adopted for the fabrication of the inertial sensor itself.
In addition, this second known solution is valid for all sensors, whether open loop sensors or closed loop sensors.
More in detail, the method and device described in U.S. Pat. No. 5,621,157 are implemented by means of an inertial sensor comprising one mobile electrode (rotor) and two fixed electrodes (stators), underneath which is set a service electrode (actuator also referred to as xe2x80x9cground planexe2x80x9d). By varying the voltage applied to the service electrode and keeping the voltage applied to the mobile electrode at a fixed value, a lateral force is produced that acts upon the mobile electrode along a direction parallel to the plane in which the service electrode lies. This lateral force is independent of the distance between the mobile electrode and the fixed electrodes, a distance which is markedly affected by the variations in the process of fabrication of the inertial sensor, and consequently it can be used as a reference force for the calibration of the inertial sensor itself.
Although this second solution is advantageous from a number of standpoints, it presents, however, the drawback that, at each variation in the voltage applied to the service electrode, there is produced on the mobile electrode, in addition to the lateral force, also a vertical force in a direction orthogonal to the plane in which the service electrode is set. In addition, the lateral force has a value other than zero only when different voltages are applied to the two fixed electrodes. Consequently, the method devised and the device made according to this second known solution are far from efficient in terms of conversion of electrical energy into mechanical energy, and are valid only for certain electrical configurations of the inertial sensor that is to be calibrated.
The technical problem that lies at the basis of the disclosed embodiments present invention is that of creating a semiconductor integrated inertial sensor with a calibration microactuator that is able to overcome the limitations and drawbacks referred to above in connection with the known art.
The technical problem is solved by an inertial sensor integrated in a body of semiconductor material and having a stator element and a rotor element that are electrostatically coupled together, the rotor element having a mobile mass, and a microactuator integrated in the body of semiconductor material, the microactuator connected to and coplanar with the mobile mass of the rotor element.
In accordance with another aspect of the present invention, the mobile mass is free to move in one direction and the microactuator has at least one first actuator element having at least one mobile actuator arm that is integral with the mobile mass and at least one first fixed actuator arm facing the mobile actuator arm, the mobile actuator arm and the first fixed actuator arm carrying respective multiple actuator electrodes and fixed actuator electrodes that are comb fingered together and extend in a direction substantially parallel to the first direction.
In accordance with yet another aspect of the invention, the mobile actuator electrodes extend on both sides of the mobile actuator arm, and the microactuator includes a second fixed actuator arm carrying a plurality of second fixed actuator electrodes that are comb fingered with respective mobile actuator electrodes, the first and second fixed actuator arms set on opposite sides with respect to a corresponding mobile actuator arm.